1. Field of the Invention
The present invention relates to an apparatus and a method for enhancing the visibility of color images, and more particularly, to an apparatus and a method for enhancing the visibility of color images by performing image processing on each pixel of continuously input color images in real time without additional hardware resources.
2. Description of the Related Art
Recently, the supply of and demand for a digital image photographing apparatus such as a digital still camera, a digital camcorder, and a mobile phone including a digital camera module are rapidly increasing. As a result, the functions of the digital image photographing apparatus have been increasingly diversified and sophisticated.
The digital image photographing apparatus includes an image pickup device, i.e., an image sensor that converts optical signals into electrical signals. As the performance of the image sensor is enhanced, the image sensor further expands the expression range (dynamic range) of the same images.
Generally, the dynamic range represents the difference between a maximum value and a minimum value of a luminance range that can be expressed by the sensor. While the human optic nerve does not have a limitation in dynamic range, the image sensor has a limitation in dynamic range at all times. Even though the performance of the recently developed high-performance image sensor is enhanced to have wide dynamic range, the image sensor has a limitation that is not beyond the dynamic range of a color negative film.
As described above, there has been a problem in that the digital image, photographing apparatus including the image sensor having a limitation in dynamic range acquires images in which bright regions and dark regions are distributed with wide variance in poor conditions such as backlight or the like is present or acquires images in which a background is very bright and a subject is very dark.
In order to solve the above-mentioned problem, various methods for enhancing visibility have been proposed. A hardware method and a software method have been known as a representative method for enhancing visibility. As for known algorithms (software method) for enhancing the visibility of images, there are a multi exposure synthesizing algorithm that photographs and synthesizes a single image at various illumination conditions, a histogram spreading method using histogram analysis, and a method for using a gamma curve, or the like. However, since the algorithms for enhancing visibility according to the related art consume a great deal of time to perform the operation and require complicated operations, the use of a separate microprocessor and an external memory is required in order to process the operation. In addition, the method for enhancing visibility using the histogram spreading and the nonlinear curve such as the gamma curve cannot simultaneously correct the bright portion and the dark portion of images, such that another deterioration phenomenon occurs during a process of enhancing the quality of an image. Therefore, the methods for enhancing visibility according to the related art have a problem in that it is impossible to process pixels of input color images in real time and costs are increased due to expensive external circuits. Further, there is a problem in that the methods for enhancing visibility according to the related art have a limitation in being applied to a small-sized apparatus such as the mobile phone including the camera module.
Therefore, a need exists for new technology to solve the phenomenon of the deterioration of the visibility of color images that is generally caused due to the limitation in the dynamic range of the image sensor of the digital color image apparatus in the art.